Aside from the B-, T-and null populations of human lymphocytes, two kinds of newly discovered lymphocytes possess both B- and T-cell surface markers may be important small subpopulations. One group has easily detectable complement-receptors (a B-cell marker) and the capacity to bind to sheep erythrocytes (a T-cell marker) but lacks other markers. These cells are tentatively called D-lymphocytes and are identifiable by a rosette method developed in our laboratory. The other group has easily demonstrable surface immunoglobulins, receptors for Fc portion of gamma globulin (both B-cell-markers) and receptors for sheep erythrocytes but lacks complement-receptors. We plan to launch an investigation to study the nature of those doubly -marked cells. We will define the normal levels of human peripheral lymphocyte subpopulations according to the various surface markers for the B- and T-cells. The oncology and the distribution of doubly- marked cells in lymphoid tissues may indicate the origin of those cells. Cell fractionation methods will be used to enrich and isolate those cells. The physical-chemical properties of those cells will be studied. The function of those cells in relation to the B- and T-cells will be tested by various methods including in vitro stimulations with antigens, antisera and mitogens. The degree of restriction of membrane markers to lymphocyte populations and the phenotypic expressions of cells to stimulations and cell transformation could be better understood. Investigations of those cells in immunoficiency, lymphoproliferative and malignant diseases will elucidate the significance of those lymphocytes in body economy. The nature of the diseases and those doubly-marked lymphocytes could thus be further understood.